Two-tier local government in England
Following the Local Government Act 1972, a standardized two-tier system of local government was introduced across England in 1974, with an upper level of county councils classified as metropolitan counties and non-metropolitan counties, and a lower level of metropolitan districts and non-metropolitan districts governed by district councils. It followed a similar two-tier system being introduced solely in Greater London in the London Government Act 1963, which introduced the upper-level Greater London Council and lower-level London boroughs in 1965.
Source: Wikipedia — Two-tier local government in England (CC BY-SA 4.0)